Method and engine for the obtainment of quasi-isothermal transformation in gas compression and expansion

ABSTRACT

The present invention refers to a procedure and a machine which make it possible to produce a quasi-isothermal compression or expansion process in any thermodynamic cycle consisting of such transformations. The procedure is possible owing to the fact that heat exchangers (A, B) independent of each other are used, in each of these heat exchangers (A, B) the working agent circulating intermittently in only one direction owing to the fact that the exchangers (A and B) are successively and cyclically connected to and disconnected from the volume of the working space (a).

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application of PCT/R081/00005 filedSept. 7, 1981 and based upon a Romanian application No. 102 311 of Oct.8, 1980 under the International Convention.

FIELD OF THE INVENTION

The present invention refers to a method as well as to an engine whichmake it possible to obtain a process of quasi-isothermal compression orexpansion, i.e., a process in which the temperature of the working agentkeeps nearly steady while undergoing practically insignificantvariations all during the compression or expansion processes in anythermodynamic cycle subject to such transformations.

BACKGROUND OF THE INVENTION

Some methods have been developed with a view to obtaining aquasi-isothermal compression or expansion process, according to which,in order to obtain the theoretical condition of an isothermaltransformation, i.e., the maintenance of equality between the mechanicwork received during the compression phase or yielded during theexpansion phase and the heat evacuated during the compression phase orthe heat absorbed during the expansion phase respectively, the workspace of variable size of an engine has been connected to a cooled heatexchanger, consisting of one or more heat exchange units, in series,during the compression phase and a heated heat exchanger during theexpansion phase (U.S. Pat. No. 3,867,815). This method has thedisadvantage that the volume of the heat exchangers adds to the volumeof the dead space, detemined by the constructive parameters of the workspace of variable size, thus preventing high compression ratios frombeing reached. In addition, owing to the fact that only oneheat-exchanger is used, the equality between the received or transferredmachanic work and the evacuated or absorbed heat respectively, cannot beensured at any instant, consequently, the transformation curve movessignificantly away from the theoretical isothermal curve, therebydamaging the efficiency of the cycle on the whole. Then there are alsoStirling external combustion engines built according to differentprinciples, in which, after the compression phase, the working agent iscooled inside a heat exchanger, afterwords run through a regenerator andfinally introduced into a heated expansion space (Stirling engine, by G.Walker). This type of external combustion engines has the disadvantageof not being able to reach higher compression values, thereby affectingthe general output of the engine.

SUMMARY OF THE INVENTION

According to the present invention, the above mentioned disadvantagesare eliminated, in order to obtain certain transformations as close tothe theoretical isothermal transformation as possible while preservingas high a compression or expansion ratio as possible, the volume of theheat exchangers does not add to the volume of the dead space determinedby the constructive parameters of the working space of variable sizebecause heat exchangers independent of each other, are provided ineither of which the working agent runs intermittently in only onedirection. These heat-exchangers are successively and cyclicallyconnected to and disconnected from the working spaces of variable size,the duration of the connection between this working space and one of theindependent exchangers is two phased, namely: in the isothermalcompression, during the first phase there is a flow of the working agentfrom one cooled independent heat-exchanger into a working space ofvarriable size, until the pressures in the two spaces become equal; theworking process is polytropic, the working agent in the working spaceconveying the heat to the working agent which comes from the exchanger,in the second phase the flow of the working agent is from the workingspace into the exchanger carrying the afferent heat, while the totalcompressed gas mass transmits the heat by means of the cooledindependent exchanger; in the expansion isotherm, in the first phase,the flow of the working agent is from the working space of variable sizeinto a heated independent heat exchanger until the pressures in the twospaces become equal, the working agent in the heat exchangertransmitting the heat to the working agent which comes from the workingspace in polytropic mixture and a second phase during which the workingagent flows from the heated exchanger into the working space, carryingthe afferent heat, while the total mass of the expandable working agentreceives the heat by means of the heated independent heat exchanger; theconnection to and disconnection from the working space of variable sizeof the independent heat exchangers is such that the lapse of time duringwhich there is no connection between the working space and the exchangerensures an isochoric evolution of the working agent in each exchangerwhile the heat is transferred towards the exterior during thecompression, and heat is received from the exterior during the expansionisotherm, the thermodynamic transformation curve in the compression orexpansion process appearing as a resultant of the summing up of somesuccessive polytropic sequential transformations, whose continuitypoints are situated above and below the theoretical isothermal curve,such that the negative mechanical work in the compression quasi-isothermand the positive mechanical work in the expansion quasi-isotherm arecomparable with those of the theoretical isothermal transformations, thepressure of the working agent in the independent heat exchangers whichensures the circulation of the working agent in only one direction beingensured by the working space of variable size itself, owing to aself-stocking process, until, after a P series of cycles, a necessarysteady value, self-repeatable with every cycle is reached.

According to the present invention, the rotary machine eliminates thedisadvantages mentioned above, owing to the fact that, in order tomateralize the procedure presented here above, it uses groups ofindependent heat exchangers, i.e. a group of cooled exchangers for thecompression phase and a group of heated exchangers for the expansionphase, the successive connection and disconnection between theseexchangers and the working space of variable size of the machine beingobtained by means of a plurality of connection orifices, some galleriesand pairs of windows provided both in the two distribution discs and inthe two fixed lids of the engine housing, windows placed radially andsecured tight, following a trapezoidal contour with expandable linearsegments and plurality of pipes for the coupling of the exchangersthemselves, a window which ensures the connection of the working spacewith the exchanger, in order to achieve the first phase of thequasi-isothermal tranformation process, while the second window ensuresthe connection for the second phase of the quasi-isothermaltransformation process, the space between the two groups of windowscorresponding to the groups of exchangers in the two lids secured tightwith the aid of trapezoidal shaped segments placed continuously on blindtrapezoidal contours, situated on the same diameter as the windows. Theprocedure and the machine for obtaining the quasi-isothermaltransformation used in gas compression or expansion processes present,acording to this invention, the following advantages:

they ensure thermodynamic transformations as close to a theoreticalisothermal transformation as possible;

they permit high compression or expansion ratios;

they ensure operation of the thermal machine at the highest possibleefficiency for the same difference in temperature, as can be achievedwith any cycle, the Carnot cycle included;

they permit any heat source to be used, such as geothermal or solarsources, as well as any type of gaseous, liquid or solid fuels;

they ensure a decrease in the fuel consumption, reducing the chemicaland phonic pollution and;

they permit the thermic machine to be operated at low pressures andtemperatures of the working agent, thus ensuring a decrease in thestress and wear level.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a diagrammatic section transverse to an axis of an engineshowing compression or expansion processes;

FIG. 2 is a pressure-volume diagram of the quasi-isothermal compressionor expanssion processes;

FIG. 3 is a temperature-entropy diagram of the quasi-isothermalcompression or expansion processes;

FIG. 4 is a theoretical pressure-volume diagram of the cycle of anexternal combustion rotary engine;

FIG. 5 is a longitudinal section of an external combustion engineaccording to invention;

FIG. 6 is a cross-section of the engine along line I--I of FIG. 5.

FIG. 7 is a detail of the sealing of the windows t and u; and

FIG. 8 is a cross-section of an engine taken along line II--II of FIG.5;

SPECIFIC DESCRIPTION

According to the present invention the method can be applied to anythermal machine that operates using a working space of variable size awhich can be successively and cyclically connected to and disconnectedfrom two groups of independent heat exchangers of V_(a1), V_(a2), V_(a3). . . etc. size, i.e. a group of cooled independent heat exchangers ofidentical construction A, and a group of independent heatedheat-exchangers of identical construction B.

Every independent cooled heat exchanger A, used in the compressionisotherm, is composed of some heat exchange units 1, provided with awindow b for the flow of the working agent coming from exchanger A tothe working space a, and a window c for the flow of the working agentcoming from the working space a to the heat exchanger A. In the same waya heated exchanger B used in the expansion isotherm, is made up of aheat exchanger unit 2 provided with a window d for the flow of theworking agent coming from the working space a to the exchanger B and awindow e for the flow of the working agent coming from the exchanger Bto the working space a.

The working space of variable size a can be developed according to theprinciple design shown in FIG. 1, on a rotary machine C, composed of astator 3 and a rotor 4 in which glide the blades 5, example which ishowever non-limitative. The rotary machine C is provided with a suction(intake) connection 6 and a discharge connection 7, or a pressureconnection 8. Following the motion of the rotor 4 the working space ofvariable size a, whose original parameters are P₀ V₀ T₀, will besuccessively connected in compression phase with the heat exchangers Aand in expansion phase with the heat exchangers B by using the windows fprovided in the wall of the working space. The state parameters of theworking agent in the first heat exchanger A are P'₁ Va₁ T"₁.

The duration of the connection between the working space of variablesize a and the heat exchanger requires two phases. In the first phaseduring which the working agent of the heat exchanger A flows towards theworking space of variable size a, through the window b of the exchangerA and the window f in the wall of the working space, yielding togetherwith the working agent of the working space a, a polytropic mixturewhose state parameters are P_(z1), V₀ +V_(a1), T_(z1), the working agentof the working space transferring the heat to the working agent whichcomes from the exchanger.

Between the values of the original state of the two gases we have thefollowing relations:

    P.sub.0 <P'.sub.1 ; T.sub.0 >T".sub.1,

while the polytropic mixture places its state parameters as follows:

    P.sub.0 <P.sub.z1 <P'.sub.1 ; T.sub.0 >T.sub.z1 >T".sub.1.

In the second phase the closing of the window b and the opening of thewindow c occur simultaneously, the two volumes being compressedtogether, while the gas flows now from the working space to the exchangethrough the windows f and c, carrying the afferent heat to the masswhich leaves the working space.

At the same time, a part of the compression heat of the joint gasescoming from the exchanger and the working space, is evacuated throughthe walls of the exchanger to the exterior, the compression showing asub-adiabatic character. At the moment of detachment of the first cooledheat exchanger A from the working space, when the orifice c closes, thegas in the working space will be in (P₁, V₁, T₁) state and the gas inthe first cooled heat exchanger A will be in the (P₁, V_(a1), T'₁)state.

Compared to their original states, the state parameters of the two gasesfollow the relations:

the working space:

    P.sub.1 >P.sub.0 ; T.sub.1 ≈T.sub.0

and

the exchanger:

    P.sub.1 >P'.sub.1 ; T'.sub.1 >T".sub.1

As soon as the working space a detaches itself from the cooled heatexchanger A, it is connected to the next cooled heat exchanger A, wherethe process is repeated exactly as in the case of the first exchanger.The working agent in the heat exchanger A, disconnected from the workingspace, develops according to an isochore curve, exchanging heat inconditions of a steady volume all during the waiting period until it isconnected to the next working space, which finds it in such stateparameters that can be considered identical with the original parametersextant at the moment of contact with the first working space (P'₁,V_(a1), T"₁).

After having run through all the heat exchangers in number of k, theworking space a undergoes successively the states: (P₀, V₀, T₀); (P₁,V₁, T₁) . . . , (P_(k), V_(k), T_(k)) with the following relationsbetween the state parameters:

    P.sub.0 <P.sub.1 . . . <P.sub.k ;

    V.sub.0 >V.sub.1 . . . >V.sub.k ;

    T.sub.0 ≈T.sub.1 . . . ≈T.sub.k ;

That is:

    P.sub.k V.sub.k constant,

while the polytropic mixture presents the successive states:

    (P.sub.z1, V.sub.0 +V.sub.a1, T.sub.z1); (P.sub.z2, V.sub.1 +V.sub.a2, T.sub.z2) . . . (P.sub.zk, V.sub.k-1 +V.sub.zk, T.sub.zk)

with the relations:

    P.sub.z1 <P.sub.z2 . . . <P.sub.zk ;

    T.sub.z1 ≈T.sub.z2 . . . ≈T.sub.zk ;

These are the very conditions of a quasi-isothermal evolution of the gasin the working space, i.e., an reduced alternative variation on eitherside of an isothermal curve.

At the same time every heat exchanger will undergo alternatively twostates: (P'₁, V_(a1), T"₁); (P₁, V_(a1), T'₁); (P'₂, V_(a2), T"₂); (P₂,V_(a2), T'₂) . . . (P'_(k), V_(ak), T"_(k)); (P_(k), V_(ak), T'_(k));while the state parameters follow the relations:

    P'.sub.1 <P.sub.1 ; P'.sub.2 <P.sub.2 ; . . . P'.sub.k <P.sub.k ;

    T".sub.1 <T'.sub.1 ; T".sub.2 <T'.sub.2 ; . . . T".sub.k <T'.sub.k ;

    P'.sub.1 <P'.sub.2 . . . <P'.sub.k ;

    T".sub.1 ≈T".sub.2 . . . ≈T".sub.k

We emphasize the essential fact that the feeding with working agent ofthe exchangers, at working parameters, and the reproduction of theseparameters with every cycle, are carried out automatically by theevolution of the cycle itself in which the working agent is absorbed bythe suction stub 6, gradually stocking the working agent in everyexchanger at stabilized parameters, reproducible with every cycle. Thesuccession of the phenomena of absorbtion, polytropic mixture, commonevolution of the united volumes and isochore cooling of the exchangersshow a tendency to a steady equilibrium of the system, owing to amonotonous variation of the state parameters of the gas, in the workingspace as well as in the heat exchangers, towards steady limits, selfreproducible with every cycle, limits whose values will be practicallyreached after some dozens of cycles, after the machine has been started.

The above explanations are based upon a mathematical research of thephenomena, out of which we present only the final results. Thus, thelimits toward which tend the pressions P_(i) in the working space whenthis latter detaches itself from each of the exchangers, are given byequations: ##EQU1## in which, besides the notations already introducedhere above, the following have also been used:

m₁, the polytropic exponent of the mixture of the two gases;

m₂, the polytropic exponent of the common evolution of the gas in theworking space and in the exchangers; ##EQU2## the isochore evolutionfactor of the gas in the exchanger number i during the waiting periodbetween the successive contacts with the two working spaces.

If we consider that the gas in the working space of variable size mixesisothermally with the gas in the cooled heat exchanger, a hypotesis thatis not far from the reality, that is m₁ =1, the equations here under canbe literally solved and we have the following relations for thestabilized values of the pressions P₁ : ##EQU3##

The values P_(i) are finite if between the volumes in the working space(V_(i)) and the volume in the independent exchanger (V_(ai)) therelation:

    (V.sub.i +V.sub.ai).sup.m2 -β.sub.i V.sub.ai (V.sub.i-1 +V.sub.ai).sup.m2-1 >0

is maintained, thus obtaining the circulation of the working agent inthe heat exchangers A and B in only one direction, i.e. in the directionexplicitly shown here above, if between the same parameters we have therelation:

    (V.sub.i +V.sub.ai).sup.m2 -β.sub.i V.sub.ai (V.sub.i +V.sub.ai).sup.m2 <0

for the quasi-isothermal compression and

    (V.sub.i +V.sub.ai).sup.m2 -β.sub.i (V.sub.i-1 +V.sub.ai).sup.m2 >0

for the quasi-isothermal expansion.

A similar development occurs in the expansion process, where the groupof heat exchangers B make it possible that the phenomenon be describedby the same equation as here above.

The intensification of the heat transfer up to the required level of theisothermal evolution of the gas in the working space with the aid ofheat exchangers as shown in the present invention, is put into evidenceby the relations already shown, on the one hand owing to the influenceof the polytropic exponent of common evolution m₁ whose value lies inthe vicinity of the unit, and on the other hand owing to theisochoreheat exchange of the exchangers expressed by the factor β_(i) which isinferior to the unit for the compression isotherm, and superior to theunit for the expansion isotherm.

The diagrams of the quasi-isothermal compression or expansion processesrepresented in FIGS. 2 and 3 respectively show that the curve of thereal transformations q for the compression and h for the expansion occuras a resultant of the summing up of some successive polytropicsequential transformations whose continuity points i are placed aboveand below the theoretical isothermal curves j for the compression and lfor the expansion. The diagram presented in FIG. 3 shows intemperature-entropy coordinates, only the curves of the realtransformations, that is, curve n for the compression and curve o forthe expansion.

The diagram in FIG. 2 shows that the negative mechanical work in thereal compression quasi-isotherm q and the positive mechanical work inthe real expansion quasi-isotherm h, are comparable to those of thetheoretical isothermal transformations j and l.

The method referring to the quasi-isothermal transformation in gascompression or expansion processes can be applied to any working cycleof any thermic machine with a working space of variable size and withexternal heat sources, such as: compressors, external combustionengines, heat pumps, refrigerating machines, etc.

Below the method is described referring to a thermal machine which worksas an external combustion engine.

According to the present invention the external combustion rotary engineis composed of a rotating cylinder 9, in which glides a double-actingpiston 10, provided with the sealing rings 11. The double acting piston10 is set at half way of its length, with the aid of the bearings 12 ona crankpin p of a crankshaft 13 and for the sake of the mounting it iscomposed of two coupled halves r, on the separation plane of thebearings by means of the bolts 14. The crankshaft 13 lies together withits main journals q in the lateral lids 15 and 16 with the aid of therollerbearings 17 and 18 on the same axis. The rotary cylinder 9 lies onthe lateral lids 15 and 16 with the aid of the roller bearings 19 and 20which define an axis III--III perpendicular to the longitudinal axis ofthe cylinder, dividing it into two equal parts. On the crankshaft 13there is a gear wheel 21 with external teeth which gears, in a 1:2ratio, a gear wheel with internal teeth 22, fixed on the rotatingcylinder 9. In the lateral walls of the rotating cylinder 9 there arefour orifices f, communicating in twos with each of the working spacesof variable size a. Fixed on the body of the journal of the rotatingcylinder 9 there are two distribution disks 23, one on either side ofthe rotating cylinder 9. The distribution disks 23 are each providedwith two windows s whence galleries 24 start, these latter connectingwindows s to windows f in the walls of the rotating cylinder 9. Whilerotating, the distribution disks 23 together with the rotating cylinder9, make the windows s pass in front of the radial windows t and udisposed in the fixed lids 15 and 16 and placed on the same diameter asthe windows s on the moving distribution disks 23, while t and u aretightened as against s.

The windows t are used for connecting the working space of variable sizea to a heat exchanger A or B in the first phase, by means of someconnections 25, while windows u are used for connecting the same workingspace to a heat exchanger A or B in the second phase of connection bymeans of connections 26. The connection 25 represents the outlet andconnection 26 the inlet in a heat exchanger unit 1 or 2 already knownand belonging with the groups of heat exchangers A or B.

Each of the windows t and u is tightened on a trapezoidal contour withthe linear and expandable segments 27, disposed in the already knownseats in the fixed lids 15 and 16. With the same linear and expandablesegments, disposed in a continous row on blind trapezoidal contours, onthe same diameter as windows t and u are also tightened the two spacesv, situated between the two groups of windows t and u corresponding tothe groups of exchangers A and B.

On the external lids 15 and 16 are disposed, in the area correspondingto the external dead point of piston 10, windows w of the same shape andradial position as windows t and u each connected to a suction stub 6.In a similar way as windows t and u, windows w are sealed on atrapezoidal contour by means of the expandable linear segments 27. Thesuction windows w can be closed after the engine has reached the ratedwork regime by any kind of control; the control is correlated with thework parameters of the engine according to already known methods.

According to the present invention, an external combustion rotary engineworks as follows. The working gases cause the double acting piston 10 toeffect a motion of translation in cylinder 9, at the same time imposingon the crankshaft 13 and the rotating cylinder 9 a rotation around axisIII--III at a speed of rotation equal to half the speed of rotation ofthe crankshaft. The motion of translation is purely harmonic, themaximum stroke of the piston being equal to four times the distancebetween the axis of the main journal p and the axis of the crankshaft13; that is four times the excentricity of the crankpin. The totalinertia forces result in a radial force, in phase with the position ofthe crankshaft; this radial force can be balanced on the crankshaft bymeans of fixed counterweights, according to a known procedure. None ofthe inertia and pressure forces acting upon the piston yields normalcomponents between the piston and the walls of the cylinder.

The gearing of toothed wheels 21 and 22 does not participate in thetransmission of the engine torque to the crankshaft. Theoretically, themechanism is completely determined without this gearing. The gearing21-22 doubles the kinematic chain piston-crankpin and its role is tofacilitate the drive of the rotation of the cylinder when the directionof the acting forces would come under the friction cone, withoutparticipating in the transmission of the torque. The role of the gearingis consequently that of overcoming the friction in the rotating motionof the cylinder or of the inertia moment, caused by the variation in thenumber of rotations, taking over the only normal forces which could haveappeared between the piston and the walls of the cylinder and would havedetermined the rotation of the cylinder. By introducing the gear, thecontact between the piston and the walls of the rotating cylinderreduces only to the contact pressure of the rings necessary to sealing.The lubrication system of the components of the engine is generallyknown.

According to the present invention, the external combustion rotaryengine works following a Carnot cycle composed of two quasi-isotherms qand h which represent the resultant of the addition of successivepolytropic sequential transformations whose continuity points i are tobe found above and below the theoretical isothermal curves j and l andadiabatic curves x and y easily obtainable by using a generally knownexternal thermal insulation of the cylinder in the working space area.

The Carnot cycle can be obtained by means of an engine as shown in theinvention, by the fact that in the first part of the compression, theworking space of variable size a successively gets into contact with thecooled heat exchanger A along the connections 25 and 26, windows t and uin the lateral lids 15 and 16, window s on the distribution disk 23,galleries 24 and the windows f in the walls of the rotating cylinder 9,stocking part of the working agent in these exchangers and compressingin a quasi-isothermal manner the remaining working agent according tothe method described here above.

As soon as the working space of variable size a has left the cooled heatexchanger A begins the adiabatic compression of the working agent thathas been left in the working space up to the interior dead point of thepiston. For this purpose the engine is provided with a generally known,corresponding thermal insulation.

The moment the piston reaches the interior dead point, the working spaceof variable size a is connected to the heated heat exchangers B, alongthe same course as shown here above, with which an exchange of workingagent occurs in a similar way as already described, thus determining aquasi-isothermal expansion of the working agent left in the workingspace. After the working space has been disconnected from the last heatexchanger B, the working agent, left inside, undergoes an adiabaticexpansion until the suction window w opens and the working space ofvariable size a comes to depression such that it will aspirate aquantity of working agent equal to the one stocked in the two groups ofheat exchangers A and B during the previous cycle, then the cyclerepeats itself successively and alternatively for the two working spacesa. The stocking process of the working agent in the working spacearrives, after some dozens of rotations of the crackshaft, to a steadystate when the necessary aspiration reduces to zero and the suctionwindow w must be closed. After having closed window w the engine workswith the working agent in closed circuit. The mechanical work of thecycle and the power of the engine increase in proportion with theincrease in the aspiration pressure of the engine.

The aspiration of the working agent can be carried out directly eitherfrom the atmosphere or from a closed tank, in which case, the stateparameters of the working agent can differ in value from the atmosphericparameters. The working agent may be any gas, gas mixture or agas-liquid heterogenous mixture. The cooling of the heat exchangers Acan be carried out in a usual way by using any cooling agent while theheating of the heat exchanger B can be obtained by using any heatsources including geothermal water, solar sources, nuclear energy or afuel burner of any type.

The given example concerning a thermal machine built according to thisinvention is not limitative. If, according to the invention, a thermalmachine were to work as a compressor, in comparison with the examplealready described, the group of heat exchangers B and the dischargeconnection 7 should be suppressed, preserving the heat exchangers A andthe enlarged suction stub 6, while a pressure connection 8 would beused. A thermal machine as shown in the invention, which were to work asa compressor, could compress the gas in a single stage at relativelyhigh compression ratios, rejecting the compressed gas at temperaturesneighboring those of the environment. A compressor working according tothe invention, owing to the rather low temperature in the compressionspace, can use synthetic materials for the piston, the segments, thevalves, etc., needing a relatively simple construction and much reducedweight and dimensions, owing to the elimination of the intermediatecompression stages.

If a thermal machine, as shown in the invention, were to work as heatpump or refrigerating machine, only the disposition of the two groups ofheat exchangers should be modified in such way as to obtain adevelopment of the cycle in opposite direction as compared to its workas an external combustion engine. A group of heat exchangers B would bethe heat source and it would represent that part of the pump whichsupplies the heat, while the other group of heat exchangers A wouldrepresent that part of the refrigerating machine which could ensure thecooling.

The procedure and the machine for the obtainment of a quasi-isothermaltransformation in gas compression or expansion processes can be appliedin any industrial domain supposed to necessitate a compression orexpansion isotherm such as chemical, refrigerating industries, etc., aswell as in any technical domain for which thermodynamic transformationsare needed in order to obtain mechanic energy, these latter being apt tobe used in transport, electric power production domains, as well as inother fields.

I claim:
 1. A method of operating a thermal machine which comprises ineach cycle of displacement of a movable member relative to a stationarymember defining a variable-volume chamber with said movable member:(a)communicating said chamber with a cooled heat exchanger through oneorifice thereof and then communicating said chamber with said cooledheat exchanger through a second orifice thereof; (b) repeating step (a)with a succession of such cooled heat exchangers while progressivelyaltering the volume of said chamber as said movable member is displacedrelative to said stationary member; (c) thereafter communicating saidchamber with a heated heat exchanger through a first and a secondorifice thereof in succession as said chamber is swept therepast withmovement of said movable member relative to said stationary member; (d)repeating step (c) with a number of heated heat exchangers in successionwhile progressively changing the volume of said chamber as said movablemember is displaced past said heated heat exchangers; and (e)controlling the work of said movable member and the communication ofsaid chamber with heat exchangers to maintain the expansion andcompression at said chamber substantially quasi-isothermal.
 2. A thermalmachine comprising:a stationary member provided with an array of cooledheat exchangers disposed along a closed path with each having a pair oforifices opening in succession along said path and a plurality of heatedheat exchangers each having a pair of orifices opening in successionalong said path; and a movable member displaceable relative to saidstationary member and defining a chamber of variable volume, saidmovable member being provided with an opening communicating insuccession with the orifices of said cooled heat exchangers and with theorifices of said heated heat exchangers to maintain a substantiallyquasi-isothermal condition during expansion and compression in saidchamber.